Grease and flour a tube pan or bundt pan. Preheat oven to 350 degrees F
Wash and dry 10-12 fresh blackberries and slice into small pieces-- You may notice a white "core" in the center- it is edible and fairly soft, but you can slice around it if you'd like. Set sliced berries to the side.
In a separate bowl, whisk the all purpose flour, salt, and baking powder for 30 seconds to combine. Set aside for later.
In a measuring cup or small bowl, add the milk, lemon juice, lemon extract, and lemon zest. Set aside for later.
Cream the softened butter with the paddle attachment in your mixer on medium speed until smooth. Gradually add the sugar and mix until lightened in color and fluffy 4 to 5 minutes.
Add the eggs one at a time, mixing after each until the yellow of the yolk blends in.
With the mixer on low speed, add the flour mixture alternately with the milk mixture. Begin and end with the dry ingredients. We do three additions of the flour mixture and two of the milk mixture. Mix just until blended. * Don't add to pan yet.
Scoop ⅓ cup of blackberry jam into a small bowl and give it a good stir to loosen it a bit.
Add ½ cup of cake batter to the bowl of blackberry jam and stir to combine. I also added a touch of red coloring gel to brighten the color a bit, but this is optional. Stir the sliced blackberries.
Scoop about half of the lemon batter (approximately 3.5 cups or so) into the bottom of the prepared bundt pan and lightly smooth it with the back of a spoon to level (or tap the side of the pan to level it).
Next, layer on the blackberry batter slightly smoothing to even out. You can sprinkle on a few sliced blackberries if you see any areas that need it. I think it is best to keep the fresh blueberries from touching the side of the pan when possible.
Layer on the remaining lemon batter. Tap the sides of the pan to smooth/level everything out, or gently smooth the top with the back of a spoon.
Bake at 350 degrees for 60-70 minutes or until a toothpick comes out clean (or with just a few crumbs attached.) Baking times can vary and so peek in as it approaches the 1 hour mark. If the top of the cake begins to get too dark, cover loosely with a piece of aluminum foil. (Ours took closer to 70 minutes to finish baking).
This recipe makes approximately 8 cups of batter.
Allow to cool in the pan for about 10 minutes before turning out onto serving plate.